Fix problem with submodules on Windows

On Windows, `repo.create_submodule(...)` failed because git didn't recognize
the worktree path set in `.git/modules/sub/config` (`fatal: bad config file
line 6 in ./config`). This commit makes `_write_git_file_and_module_config`
convert the worktree path to the linux format (forward slashes) which git
recognizes.

Author: msiemens

git/objects/submodule/base.py

@@ -293,7 +293,8 @@ def _write_git_file_and_module_config(cls, working_tree_dir, module_abspath):
         fp.close()
 
         writer = GitConfigParser(os.path.join(module_abspath, 'config'), read_only=False, merge_includes=False)
-        writer.set_value('core', 'worktree', os.path.relpath(working_tree_dir, start=module_abspath))
+        writer.set_value('core', 'worktree',
+                         to_native_path_linux(os.path.relpath(working_tree_dir, start=module_abspath)))
         writer.release()
 
     #{ Edit Interface